These layers show some differences in how WMS requests get
rendered, leading me to wonder if there was a Geoserver code
change that may have led to this observed behavior.

To be more specific:

We store thumbnail (low resolution, 300x300) images of all of our
layers. When we make configuration changes, we make wms requests
that we compare against those thumbnails. Any found differences
get flagged for review (in this case, all of our polar
stereographic north layers). When I discovered that there was a
problem upgrading from Geoserver v2.11.5 to v2.14.1, I did some
additional testing.

'native' resolution wms requests are consistent. In other
words, if I request an image with a bounding box and size (in
pixels) of the matching source data file on disk, the resulting
image is the same in both v2.11.5 and v2.14.1

Other WMS requests (e.g., 300x300 thumbnails, or double the
native resolution), result in differences in a grid-like
pattern. See the attachments vel_thumb_diff.png and
conc_double_diff.png, which show the grid-like pattern of
differences for two of our layers. These images were created
using imagemagick's `compare` utility (each red pixel represents
a difference between wms responses in v2.11.5 and v2.14.1).

All of our layers are image mosaics with the TIME dimension
enabled. I tried adding a single GeoTiff from one of the affected
layers as a GeoTiff store, and it did not experience this
behavior. Appears as though this observed behavior only affects
image mosaics.

To reproduce this issue, see the attached 'steps_to_reproduce.txt'

Given that differences seem to only appear when an image is
requested that does not match the 'native' resolution of the
source file on disk, I suspect this might have something to do
with differences in how image interpolation occurs between these
versions.

GEOS-5482 (https://osgeo-org.atlassian.net/browse/GEOS-8542),
which resolved an issue with polar stereographic projections may
be related. I have not seen anything else obvious in the
changelogs for releases since v2.11.5 which would suggest WMS
rendering changes for polar stereographic projections, but I may
have missed something. Does anyone know if what I am seeing is
expected, or might this be a bug?

Re: Polar stereographic north WMS rendering (Image mosaics)

I have done a bit more experimentation and have found a similar
rendering issue in a geoserver instance upgrading from v2.10.2 to
v2.11.5. This time an EPSG:4326 layer is affected while the polar
stereographic layers are not. An examination of the differences
appears to suggest once again that this is probably a difference
in how interpolation is performed between these versions.

I suppose this brings up the question of if the regression tests
I am performing should ever be expected to be consistent? Given
that requesting 'full resolution' images provides the same result
(no interpolation) between the versions I have tested, I could go
that route for our tests. This would be at the trade-off of
storage space, but our main concern is preventing a scenario in
which a Geoserver upgrade results in a layer not being rendered at
all (we had some major issues with our polar stereographic layers
before, see https://osgeo-org.atlassian.net/browse/GEOS-8542).

Does anyone have any thoughts on this? I'd appreciate any
feedback!

Thanks again,

Trey

On 12/19/18 1:38 PM, Trey Stafford
wrote:

Hello!

We are upgrading our Geoserver instance from version 2.11.5 to
2.14.1. As part of that task, we have been running regression
tests to ensure that our data are still served and rendered as
expected.

These layers show some differences in how WMS requests get
rendered, leading me to wonder if there was a Geoserver code
change that may have led to this observed behavior.

To be more specific:

We store thumbnail (low resolution, 300x300) images of all of
our layers. When we make configuration changes, we make wms
requests that we compare against those thumbnails. Any found
differences get flagged for review (in this case, all of our
polar stereographic north layers). When I discovered that there
was a problem upgrading from Geoserver v2.11.5 to v2.14.1, I did
some additional testing.

'native' resolution wms requests are consistent. In other
words, if I request an image with a bounding box and size (in
pixels) of the matching source data file on disk, the
resulting image is the same in both v2.11.5 and v2.14.1

Other WMS requests (e.g., 300x300 thumbnails, or double the
native resolution), result in differences in a grid-like
pattern. See the attachments vel_thumb_diff.png and
conc_double_diff.png, which show the grid-like pattern of
differences for two of our layers. These images were created
using imagemagick's `compare` utility (each red pixel
represents a difference between wms responses in v2.11.5 and
v2.14.1).

All of our layers are image mosaics with the TIME dimension
enabled. I tried adding a single GeoTiff from one of the
affected layers as a GeoTiff store, and it did not experience
this behavior. Appears as though this observed behavior only
affects image mosaics.

To reproduce this issue, see the attached
'steps_to_reproduce.txt'

Given that differences seem to only appear when an image is
requested that does not match the 'native' resolution of the
source file on disk, I suspect this might have something to do
with differences in how image interpolation occurs between these
versions.

GEOS-5482 (https://osgeo-org.atlassian.net/browse/GEOS-8542),
which resolved an issue with polar stereographic projections may
be related. I have not seen anything else obvious in the
changelogs for releases since v2.11.5 which would suggest WMS
rendering changes for polar stereographic projections, but I may
have missed something. Does anyone know if what I am seeing is
expected, or might this be a bug?